1. Field of the Invention
The present invention relates to an electrode for the generation of plasma arcs used in plasma machining devices, and in particular, to an electrode having a heat-resistant insert made of hafnium, zirconium or alloys thereof, and to an improvement of the electrode structure with the purpose of improving the durability of the electrode.
In particular, the present invention prolongs the electrode lifetime in oxygen plasma cutting which is useful for cutting mild steel.
2. Description of the Related Art
Electrodes made of highly heat-resistant metals, such as tungsten (W), hafnium (Hf) or zirconium (Zr) are used in plasma machining devices, especially in plasma cutting devices. When the temperature of the electrode exceeds 3000xc2x0 C. during the arc generation, it thermally emit electrons and operates as a cathode spot. Such electrodes can be broadly classified into two types, depending on their material. The first type uses tungsten, or tungsten into which small amounts of other elements have been added. The second type uses hafnium or zirconium.
These two types of electrodes are also used with different plasma gases (working gas: the gas that is turned into a plasma by the arc emission). Tungsten electrodes are used in plasma cutting devices using argon (Ar), helium (He), nitrogen (N2) or hydrogen (H2) either alone or as gas mixtures as the plasma gas. On the other hand, hafnium or zirconium electrodes are used in plasma cutting devices using oxygen or air as the plasma gas. That is to say, tungsten electrodes are used when the plasma gas does not contain oxygen, and hafnium or zirconium electrodes are used when the plasma gas contains oxygen. The reason for this is that tungsten alone has a very high melting point (about 3400xc2x0 C.) and boiling point (about 5700xc2x0 C.), but when it oxidizes, the melting point and the boiling point are lowered considerably (the melting point to about 1500xc2x0 C. and the boiling point to about 2000xc2x0 C.), so that it cannot be used as an insert anymore. In contrast, the melting point of hafnium and zirconium alone is a little lower (about 2200xc2x0 C. for hafnium), but the melting point of their oxides is actually higher (about 2800xc2x0 C. for hafnium), so that they can be used satisfactorily as inserts.
Depending on the material to be cut by plasma cutting, there are optimal plasma gas combinations for attaining a favorable cutting quality. Especially for mild steel cutting, which occupies a large proportion of applications for plasma cutting, oxygen plasma-cutting, in which a plasma containing oxygen is used, attains the best cutting quality, and has an excellent cutting speed.
The thermal conductivity of hafnium (the following explanations relate to hafnium, but the same is true for zirconium), which is the electrode material for oxygen plasma cutting, is very poor and is only one tenth of that of copper, so that if the electrode is made of hafnium alone, it is usually not cooled enough, the temperature of the hafnium rises too much, and the consumption of the hafnium may proceed rapidly. In order to prevent this, in electrodes using hafnium, usually a substantially cylindrical electrode body is made of copper, and a substantially column-shaped small piece of hafnium (referred to as xe2x80x9cinsertxe2x80x9d in the following) is inserted into a tip, which serves as the cathode spot, of the cylindrical copper electrode body (referred to as xe2x80x9cholderxe2x80x9d in the following). The cylindrical copper holder is cooled by air or by water, so that the hafnium insert in its tip is. cooled due to the thermal conduction with the copper holder.
Thus, for oxygen plasma cutting, electrodes are used that have hafnium or zirconium inserts in their tips. However, since the temperature of the cathode spot exceeds 3000xc2x0 C. during the plasma arcandgeneration, it is difficult to reduce the consumption of the hafnium or zirconium to the point where it is negligible, even using materials formed of high melting point oxides, such as hafnium oxide or zirconium oxide. Thus in the past, several techniques have been developed to reduce the electrode consumption and improve the lifetime of electrodes.
For example, the thermal shock to the electrode can be dampened by slowly increasing the arc electric current immediately after the arc ignition, which reduces the electrode consumption right after the arc ignition (see JP H05-104251A). Or, the electrode consumption immediately after the arc ignition is reduced by igniting a plasma arc with nitrogen and then switching to oxygen plasma (see JP H03-258464A). Another method that has been proposed is to reduce the electrode consumption by optimizing the insert diameter with respect to the arc electric current (see JPH07-506772A). A further method that has been proposed is to accelerate the cooling of the insert and improve the electrode lifetime by forming an intermediate layer of a silver alloy between the insert and the holder to improve the thermal conduction between the insert and the holder (see JP H04-167996A).
However, in spite of those technical improvements, the durability of electrodes is limited to a few hours in actual oxygen plasma cutting, and there is great demand for a further increase of their lifetime.
FIG. 1 shows schematically how the electrode is consumed away during the generation of an arc. The arc generation first consumes the insert 11 at the tip of the electrode 10, until it is shaped like a mortar (FIG. 1(b)). The speed with which the insert 11 is consumed varies with such factors as the current, the cooling of the electrode 10, the composition of the plasma gas, and the gas pressure. Moreover, as the.arc generation proceeds, the consumption of the insert 11 invades deeper to make a hole in the tip of the electrode 10 (FIG. 1(c)). Then, when the consumption depth d of the insert 11 (that is, the distance from the top surface of the consumed insert 11 to the top surface of the electrode 10) reaches a limit value dmax, a stable arc emission from the insert 11 becomes impossible, and arc generation becomes difficult, the arc starts to be emitted from the copper holder 12, and the copper holder 12 is consumed rapidly, which leads to destruction of the electrode 10 (FIG. 1(d)).
One might think that if the consumption speed of the electrode stays the same, it should be possible to prolong the possible usage time of the electrode by increasing the volume of the insert. However, if the diameter D of the hafnium insert 11 is simply increased, then the thermal conduction of the insert 11 worsens, so that the temperature inside the insert 11 rises and the consumption speed accelerates more than what the volume has been increased, thereby instead rather shortening the electrode lifetime. That is to say, there is an optimal value for the diameter D of the insert, and there is no advantage in simply enlarging it (for an invention related to the optimization of the insert diameter, see JP H07-506772A) Also even when the buried length H of the insert 11 is increased more than the limit depth dmax, the consumption does not proceed beyond the limit depth dmax. The limit depth dmax, which depends on the swirling of the plasma gas stream, the cooling of the electrode and the arc electric current, is usually about 1 mm to 2 mm and does not depend on the buried length H of the insert 11. Consequently, it is sufficient if the buried length H of the insert 11 is equal to the limit depth dmax at least. There is no advantage in making the buried length H larger than the limit depth dmax, but this is uneconomic, because the expensive hafnium is used in excess.
Consequently, it is an object of the present invention to improve the electrode structure of a plasma machining electrode having a hafnium or zirconium insert, so as to prolong the electrode lifetime.
It is another object of the present invention to improve the arc generation conditions for this improved plasma machining electrode, so as to prolong the electrode lifetime.
A plasma machining electrode in accordance with the present invention includes a holder serving as an electrode body, and an insert inserted into a tip of the holder and joined therewith. The material of the insert is selected from group consisting of hafnium, zirconium, hafnium alloys and zirconium alloys. The insert has a protruding portion that protrudes from a tip face of the holder. The protruding portion makes the insert longer and increases the volume of the insert subject to consumption, so that the electrode lifetime is prolonged.
In a preferable embodiment, the insert is substantially cylindrical, and the protrusion length that the protruding portion of the insert protrudes from the holder top surface is not more than the diameter of the insert, for example, not more than 0.5 mm.
In a preferable embodiment, the protruding portion of the insert has a rounded profile without sharp angles.
In a preferable embodiment, the insert and the holder are joined together by a metallurgical method, such as silver brazing, considering favorable thermal conduction, but it is also possible to use a mechanical junction like pressure welding or forcing.
In a preferable embodiment, water cooling, in which cooling water flows inside the electrode, is used to improve the cooling, and the insert pierces the tip of the holder, so that the rear surface of the insert is exposed to the cooling water flow inside the electrode.
In a preferable embodiment, using the electrode as a plasma machining electrode, a plasma gas containing at least 5 mol % nitrogen is used at least when starting a first arc. To increase the electrode lifetime for oxygen plasma cutting to the maximum, a plasma gas of pure nitrogen can be used for starting.the arc, and for the main arc transition, a mixed gas of 75 to 95 mol % oxygen and 25 to 5 mol % nitrogen can be used.
Incidentally, there are conventional tungsten electrodes in which the insert protrudes from the tip face of the holder. However, in electrodes with hafnium (or zirconium) inserts, there is the following critical problem with protruding insert structures, so that they were believed to be impractical. The problem is rooted in the fact that the characteristics of tungsten and hafnium electrodes are different. More specifically, in thermal electron emission during the generation of an arc, the electrode surface of tungsten electrodes is solid, except for the vicinity of the cathode spot where the temperature is highest. In hafnium or zirconium inserts, however, a considerable portion of the insert surface is assumed to be liquid. Therefore, if the hafnium or zirconium insert protrudes from the holder, the protruding portion becomes liquid, so that this portion may be blown off. When. this blowing off of the insert occurs, the blown off portion adheres to the inside wall of the nozzle facing the electrode, which upsets the stable flow of the plasma gas and becomes a reason for cutting defects. Depending on the circumstances, it may even become a reason for arc instabilities that lead to the immediate destruction of the electrode. Therefore, to retain the liquid hafnium, in electrodes using conventional hafnium inserts, the surface of the inserts of new electrodes is coplanar with the tip face of the holder or even somewhat depressed toward the inside. The reason for depressing the insert surface below the holder surface is that when generating the first arc with a new electrode, more hafnium is consumed than at the second or further arc generations. This is, because there is no hafnium oxide formed on the hafnium surface of new electrodes, so that the consumption speed is high. Therefore, the cooling of the insert is improved by depressing the insert surface, with the goal of reducing the initial consumption. But in any case, it was conventionally believed that for electrodes with hafnium or zirconium inserts, it is not possible to let the insert protrude from the copper holder.
The present invention is revolutionary, in that it runs completely counter to that commonly held conventional belief. That is to say, as the result of many experiments performed by the inventors, it has been found that if the hafnium or zirconium insert protrudes outward, this protruding portion is not necessarily blown off by the arc generation, but may be retained at the electrode tip, which is a completely novel insight, and the present invention is based on this insight. With further experimentation performed by the inventors, several specific conditions have been pursued, under which the protruding portion of the insert is retained reliably at the electrode tip, and the volume of the protruding portion contributes to the lengthening of the electrode lifetime. These conditions are reflected by the preferred embodiments explained in detail below.